1. Field of the Invention
The present invention is concerned with a process and apparatus that separates alkali metal sulphide from alkali metal hydroxide in kraft pulping liquors using a system incorporating an amphoteric type ion-exchange resin.
2. Description of Prior Art
Lignin removal by pulping is more economical and more environmentally friendly than by bleaching. Pulping to a lower kappa number is, however, associated with a loss in viscosity of the pulp. Processes such as MCC--modified continuous cooking (Johansson et al., Svensk Papperstidning nr 10, 30-35, 1984) and EMCC--extended modified continuous cooking (Jiang et al., APPITA, 45 (1), 19, 1992) have been developed to improve selectivity and maintain viscosity. It is recognized that the selectivity of the process could be improved further by applying a high sodium sulphide to sodium hydroxide ratio at the beginning of the delignification phase (Norden, S. and Teder, A., Tappi, 62(7), 49-51, July 1979; Teder, A. and Olm, L., Paperi ja Puu 63(4a), 315-322, 1981; Mao, B. and Hartler, N., Paperi ja Puu--Paper and Timber, 74(6), 491-494, 1992). It is also recognized that a low concentration of dissolved lignin and a high alkali concentration in the final delignification phase increase the lignin removal rate (Sjoblom et al., Paperi ja Puu -Paper and Timber, 5, 452-460, 1988; Mao, B. and Hartler, N., Nordic Pulp and Paper Research Journal, 4, 168-173, 1992). Based on these findings, it is expected that the pulping process would be improved through processes that generate a sulphide-rich and a sulphide-poor liquor while maintaining the sulphur balance in the kraft process.
Extended modified cooking has also been examined in the presence of polysulphide which increases pulp yield. Extended modified cooking and polysulphide pulping are two compatible processes that offer complementary advantages (J. E. Jiang, Tappi, 77(2), 120-124, February 1994). A process that provides a high polysulphide concentration in the presence of a low concentration of hydroxide would allow further improvements of the pulping process. Polysulphide liquor is prepared from white liquor by using an activated carbon catalyst (Nakamura, M. and Ono, T., Proceedings of the Tappi Pulping Conference, Atlanta, 407, 1988; Lightfoot, W. E., Pulp and Paper, 64(1):88, 1990), or using a lime mud catalyst in the presence of MnO.sub.2 (Dorris, G. M., U.S. Pat. No. 5,082,526, Jan. 21, 1992).
At present, there are five published methods of producing liquors with different sulphidities, although none has been tried on a commercial scale. These are the white liquor preparation method by H. A. Simons Ltd. (Lownertz, P. P. H., International Patent Application, W092/20856, 26 Nov. 1992), the Desulphur and Green Liquor Cooling Crystallization processes by Ahlstrom (Ryham, R. and Lindberg, H., 80th Annual Meeting, CPPA Technical Section, B179-B190, Feb. 1994), black liquor gasification (Grace, T. M. and Timmer, W./M., Pulp and Paper, 69(11):49 (1995)), and separation of sulphides and chlorides from pulping liquors by electrodialysis by Paprican (Thompson et al, U.S. Pat. No. 5,536,384, Jul. 16, 1996 and International Patent Application, Wo 96/25225, Aug. 22, 1996).
Apart from pulping, white liquor is also used as a source of alkali in oxygen delignification. The white liquor used is oxidized prior to oxygen delignification to convert the Na.sub.2 S content to Na.sub.2 S.sub.2 O .sub.3 (partial oxidation) or Na.sub.2 SO.sub.4 (full oxidation). Since the effluent from an oxygen delignification stage is compatible with the kraft recovery cycle and can be recycled to the recovery boiler, oxidized white liquor is used in place of purchased caustic soda to maintain the mill sodium/sulphur balance. If oxidized white liquor is used as a source of alkali in bleaching extraction stages, peroxide bleaching and/or brightening, alkaline leaching (Li, j and MacLeod, J. M., Journal of Pulp and Paper Science, 19(2):j85 (1993)) or scrubbing applications, complete oxidation of alkali metal sulphide to alkali metal sulphate is required (Ayala etal, Tappi Oxygen Delignification Symp., Toronto, Notes: 153-161, Oct.17-19, 1990). This is to prevent the consumption of bleaching chemicals and the emission of TRS gases in scrubbing applications. Complete oxidation of white liquor involves considerable capital and operating costs. The present invention provides an efficient way to separate alkali metal sulphide from white liquor without the production of alkali metal thiosulphate or sulphate. The resulting caustic soda solution is compatible with the bleaching and scrubbing operations. Furthermore, the operating costs of such a process are much lower than the oxidation of white liquor, since it uses water to achieve the separation.
Amphoteric ion-exchange resins also referred to as snake-cage polyelectrolyte resins operate on the principle of ion retardation and are particularly useful in removing salts from aqueous solutions of non-electrolytes, and for separating metal ions or anions. A snake-cage resin is a cross-linked polymer system containing a physically trapped linear polymer "a snake". Because of its high molecular weight, the snake cannot diffuse out of the highly cross-linked cage. The snake-cage resin corresponds to a microscopically mixed ion-exchange bed, the mixing being on the molecular level, (Hatch et al, Industrial and Engineering Chemistry, 49(11), 1812-1819, Nov. 1957). The cation-exchange sites of the resin are very close to the anion-exchange sites--probably in the range of 10 A or less. Thus the resin system is already internally neutralized. Therefore, if sodium chloride is fed through a column containing such a resin, the resin sorbs both the sodium and chloride ions simultaneously, weakly and reversibly. Sorbed salts can be removed from such a resin by merely washing with water. When this resin is used in a resin bed, many water-soluble ionic and non-ionic materials can be separated merely by feeding a certain volume of the solution to the amphoteric resin bed and subsequently rinsing the bed with water. As the ionic materials are weakly sorbed, they appear in the later fractions of the eluate. Since the movement of ionic materials on the resin bed is retarded, this separation method is called ion retardation. Such a separation approach corresponds to a many-stage fractionation, and often can separate ionic and non-ionic materials very efficiently. The ionic materials can diffuse freely throughout all the water-swollen inner region of the resin particles and the capacity is very much greater than would be the case if only surface sites were involved.
These resins are often many times more selective for one ionic species than for another. These differences in ion-selectivities often make it possible to fractionate mixtures of ionic materials. For example, the polyacrylate Dowex (trade-mark of Dow Chemical) 11A8 snake-cage resin is much more selective for sodium chloride than for sodium hydroxide. This selectivity can be exploited in removing sodium chloride--a contaminant--from caustic solutions containing sodium chloride. Although the high selectivity of the snake-cage resin for chloride has been documented before (Hatch et al, Industrial and Engineering Chemistry, 49 (11), 1812-1819, Nov. 1957), there are no published data on the selectivity of this resin to sodium sulphides.
Although amphoteric ion-exchange resins are normally used in particulate form, an amphoteric ion-exchange resin may be made in the form of a membrane for use in diffusion dialysis. In such a case, water is used to strip sulphide/polysulphide from the membrane. The diffusion dialysis process using amphoteric ion-exchange membrane is the equivalent of ion-retardation in a fixed bed.